The present invention relates to novel polymeric compounds, binder resin composed of the polymeric compounds, and polymer electrolyte compositions having a high ionic conductivity and tackiness, and secondary cells.
Electrolytes used in electrochemical devices such as secondary cells have until now been primarily low-molecular-weight substances that are liquid at or above room temperature, such as water, ethylene carbonate, propylene carbonate, and tetrahydrofuran. In lithium-type cells in particular, use is commonly made of low-molecular-weight organic liquid electrolytes which tend to evaporate, ignite and burn rather easily. To ensure long-term stability, a metal can must be used as the outer cell enclosure so as to increase the airtightness of the container. The result has been a considerable rise in the weight of electrical and electronic components which use low-molecular-weight organic liquid electrolytes, and a complex production process.
By contrast, the use of a polymer as the electrolyte allows electrolytes to be obtained which have a very low volatility and are not prone to evaporation. Moreover, such xe2x80x9cpolymer electrolytesxe2x80x9d that have a sufficiently high molecular weight can even be used as solid electrolytes which are not fluid at or above room temperature. They offer the dual advantage of serving as a solvent for ion-conductive salts and of solidifying the electrolyte.
As an example of this type of polymer electrolyte, in 1978, Armond et al. at l""Universitxc3xa9 de Grenoble in France discovered that lithium perchlorate dissolves in solid polyethylene oxide, and reported that when 1 M lithium salt is dissolved in polyethylene oxide having a molecular weight of about 2,000, the resulting complex exhibits an ionic conductivity at room temperature of about 10xe2x88x927 S/cm. Another group reported that when 1 M lithium salt is dissolved in polyethylene oxide having a molecular weight of about 200, which is liquid at room temperature, the ionic conductivity at room temperature is about 10xe2x88x924 to 10xe2x88x925 S/cm. Thus, it is known that polymeric substances such as polyethylene oxide with the ability to dissolve ion-conductive salts function as electrolytes.
Since then, similar research has been carried out on a broad range of largely polyethylene oxide-related polymers, such as polypropylene oxide, polyethyleneimine, polyurethane and polyester.
As noted above, polyethylene oxide, the most thoroughly investigated of these polymers, has a good ability to dissolve ion-conductive salts. However, because it is a semi-crystalline polymer, when a large amount of metallic salt is dissolved therein, the salt forms a pseudo-crosslinked structure between the polymer chains that leads to crystallization of the polymer. As a result, the actual ionic conductivity is much lower than the predicted value.
This is because an ion conductor dissolved in a linear polyether-based polymer matrix such as polyethylene oxide migrates with the local movement of polymer chain segments within amorphous regions of the polymer matrix. With the formation of a pseudo-crosslinked structure, the cations which carry the ionic conductivity are strongly coordinated by the polymer chains, greatly reducing cation mobility and thus lowering the conductivity. Such local movement of the polymer chains is referred to as Brawnian motion.
For this reason, a linear polyether-based polymer such as polyethylene oxide is a poor choice as the matrix polymer for an ion-conductive polymer electrolyte.
In fact, according to the literature to date, ion-conductive polymer electrolytes composed entirely of linear polymers such as polyethylene oxide, polypropylene oxide and polyethyleneimine generally have an ionic conductivity at room temperature of about 10xe2x88x927 S/cm, and at best not higher than about 10xe2x88x926 S/cm.
To obtain ion-conductive polymer electrolytes having such a high conductivity, a molecule must be designed which allows the existence within the matrix polymer of many amorphous regions of good ion conductor mobility, and which does not crystallize even when an ion-conductive salt is dissolved therein to a high concentration.
One such method is the attempt, described by N. Ogata et al. (Sen""i Gakkaishi, pp. 52-57, 1990), to introduce a branched structure into polyethylene oxide. Their work demonstrates that polyethylene oxide derivative-based ion-conductive solid polymer electrolytes having a high ionic conductivity (about 10xe2x88x924 S/cm at room temperature) can indeed be synthesized. However, such polymer electrolytes have not been commercialized on account of the sheer complexity of the polymer synthesis method involved.
The inventor previously disclosed that polymer electrolyte-forming polymers having a high ionic conductivity can be prepared by introducing polyoxyalkylene branched chains onto a natural polymeric substance such as cellulose and capping the terminal hydroxyl groups with suitable substituents, and that such polymers can be used to form solid polymer electrolytes having an excellent strength and a high conductivity (JP-A 8-225626 and JP-A 9-50824).
However, solid polymer electrolytes in which polyoxyalkylene branched chains have been introduced onto a natural polymeric substance such as cellulose have two drawbacks: (1) because the molecular weight per polymer chain (backbone) unit is high, any further increase in the fraction of polyoxyalkylene segments, which is where ion-conductive salt dissolution and migration takes place, per unit weight of the natural polymeric substance is difficult to achieve; and (2) the tackiness tends to be somewhat poor.
The present invention was conceived in light of these circumstances. One object of the invention is to provide a novel polymeric compound; and a binder resin compose of the polymeric compound. Another object is to provide a polymer electrolyte-forming polymer composed of the novel polymeric compound. Yet another object is to provide an ion-conductive polymer electrolyte composition endowed with high ionic conductivity and high tackiness which lends itself well to use as a solid polymer electrolyte in applications such as film-type cells; and a secondary cell comprising the binder resin and the ion-conductive polymer electrolyte composition.
The inventor has conducted extensive and repeated investigations in order to achieve these aims. As a result, the inventor has discovered that an effective way to raise the ionic conductivity within a polymer electrolyte-forming polymer is to increase the proportion per unit weight of polymer electrolyte-forming polymer in which polyoxyalkylene segments capable of dissolving an ion-conductive salt are introduced onto the polymer.
That is, a typical example in which polyoxyalkylene branched chains are introduced onto a conventional natural polymeric substance such as cellulose might involve the introduction of a 10-mole unit length polyoxyethylene group per cellulose unit. In this case, the molecular weight of the cellulose recurring units (C6H10O5) is 162 and the molecular weight of the 10-mole polyoxyethylene groups ((CH2CH2O)10xe2x80x94H) is 441. Hence, the fraction represented by the polyoxyethylene groups, which are the portions of the polymer that dissolve the ion-conductive salt, relative to the unit weight of the resulting cellulose derivative (polyoxyethylene fraction) is given by the ratio 441/(441+161)=0.733.
By contrast, if a polymeric compound such as polyvinyl alcohol (PVA), having a unit molecular weight lower than natural polymeric substances such as cellulose is used as the backbone, given that the molecular weight of the PVA recurring units (CH2CH(OH)) is 44 and the molecular weight of the 10-mole polyoxyethylene groups ((CH2CH2O)10xe2x80x94H) is 441, a higher polyoxyethylene fraction of 441/(441+44)=0.909 is achieved. The higher polyoxyethylene fraction enables a greater amount of ion-conductive salt to be dissolved, in addition to which the molecule has a larger number of polyoxyethylene segments where ion migration occurs, increasing ion mobility. The inventor has thus found that a high ionic conductivity can be attained in this way.
Also, when a film-type cell, for example, is assembled using a solid polymer electrolyte, in order for the solid polymer electrolyte to additionally serve as the binder component for the cell, the electrolyte must have both a high ionic conductivity, and the ability to bind the powdery battery active material. That is, it must be tacky. Moreover, film-type batteries made with solid polymer electrolytes generally have a positive electrode/solid electrolyte/negative electrode construction. Unlike cylindrical batteries in which this positive electrode/solid electrolyte/negative electrode composite is coiled and placed in a can, the absence of a coiling pressure in film-type batteries means that pressure is not applied between the positive electrode and the solid electrolyte and between the solid electrolyte and the negative electrode, allowing the solid electrolyte to separate readily from the positive electrode and the negative electrode. For this reason as well, the solid electrolyte placed between the positive electrode and the negative electrode, in addition to serving as an electrolyte, must also have the ability to strongly bond the positive and negative electrodes. In other words, it must have tackiness.
Pursuing their investigations even further in light of such considerations, the inventor has found also that polymeric compounds containing polyvinyl alcohol units of general formula (1) below and having an average degree of polymerization of at least 20 in which some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3, and especially such polymeric compounds in which some or all of the hydroxyl groups on the molecule are additionally capped with one or more type of monovalent group selected from among halogen atoms, substituted or unsubstituted monovalent hydrocarbon groups of 1 to 10 carbons, acyl groups, triorganosilyl groups, amino groups, alkylamino groups and phosphorus-containing groups, have the ability to dissolve a large amount of ion-conductive salt because of their high oxyalkylene fraction. Moreover, the presence in the molecule of more oxyalkylene segments over which the ions can migrate increases ion mobility, enabling a high ionic conductivity to be achieved. Also, these polymeric compounds have a high tackiness which allows them to function well as a binder component that firmly bonds the positive and negative electrodes. The inventor has thus discovered that compositions comprising an ion-conductive salt dissolved to a high concentration in a binder resin composed of such a polymeric compound are ideally suited for use as a solid polymer electrolyte in film-type batteries and related applications.
Accordingly, the present invention provides:
(1) a polymeric compound comprising polyvinyl alcohol units of general formula (1): 
xe2x80x83wherein n represents a number of at least 20, and having an average degree of polymerization of at least 20, characterized in that some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3;
(2) a binder resin composed of the above polymeric compound;
(3) an ion-conductive polymer electrolyte composition comprising primarily the same polymer electrolyte-forming polymer (that is, the foregoing polymeric compound) and an ion-conductive salt; and
(4) a secondary cell comprising a positive electrode, a negative electrode and a solid polymer electrolyte layer, characterized in that the solid polymer electrode layer is composed of the above ion-conductive polymer electrolyte composition and lies between the positive electrode and the negative electrode.